The usage of multiple antennas and wireless systems with distributed antennas has several benefits as widely reported in literature. The exemplary use case discussed herein is the cellular communication system. The growing demand of throughput and mobility in cellular systems is changing the paradigm of radio access networks (RAN). Small cells and pervasive deployment of antennas appear the only viable solution to increase the RAN capacity in term of throughput per square-km. The centralized RAN (CRAN, shown in FIG. 1) is designed to centrally handle a huge number of antennas to simultaneously serve a large number of users in the same time-frequency resource by taking advance of mutual cooperation for interference mitigation. Antennas, together with a minimal local processing, are hosted in remote units (RUs) deployed at the cell sites or densely geographically distributed indoor/outdoor, while modulation/demodulation processing is carried out at multiple baseband units (BBUs) that are arranged remotely in the same hotel with large benefits in terms of scalability and programmability [1].
Front-hauling is the technical word used to refer to the connection between BBUs and RUs based on the principle of transporting the RU-to-BBU (uplink) and BBU-to-RU (downlink) radiofrequency (RF) signals. Conventionally, front-hauling is designed to exchange the in-phase and quadrature (IQ) streaming of the RF signals after these are digitized and arranged into a sequence of packets according to any of the routinely employed serial protocols such as CPRI [2], or any other similar protocols for digital IQ streaming. Drawback of digitalization of RF signals is the bandwidth expansion that can be as high as ×30, reduced to ×16-18 for compressed CPRI [3]. Even if optical network appears to be the only viable technology to transport several Gbps, the use of several antennas at RUs scales the bandwidth according to their number Na and the corresponding data-rate can easily rise to several tens of Gbps for each RU with multiple antennas. To exemplify, in case of reception of RF signals having 100 MHz bandwidth by one antenna, the digital front-hauling needs at least 3 Gbps (or 1.6-1.8 Gbps if compressed CPRI) for RF signals transport; for a RU with Na=10 antennas the data-rates becomes 30 Gbps (or 16-18 Gbps if compressed), and data-rate can reach hundreds of Gbps for large antennas or aggregated multiple RUs.
In contrast to digital IQ streaming, the direct relaying of the analog RF signals over optical fibers or cables (after some carrier frequency adaptations) avoids any bandwidth expansion and is considered a promising solution that has been investigated in the past also by the inventors.
Wireless or radio over copper (RoC) translates the RF carrier to fit into the bandwidth of twisted pair copper cables to be transported between RU and BBU, the transport of RF carrier can coexist with other services such as xDSL [4]. Benefit of RoC is that it can reuse the pre-existing twisted pair telephone lines, and even the LAN cables that are largely existing in buildings and enterprises, and RoC equipments can be powered over the same cables. Radio over fiber (RoF) uses a similar approach by direct modulation of the laser with RF signals [5]. However, the linearity requested by direct modulation is a severe limitation to the adoption of RoF that leave several research topic still open to the optical communication community.
Applicant has noticed that the RoC architecture is an analog-relay system that offers several benefit, collected in table 1 here below, compared to the digital IQ streaming and RoF. Namely, RoC uses similar RF electronics for up (BB-to-RF) and down (RF-to-BB) conversion to baseband (BB), and the circuitry for bandwidth translation (mixers) are of modest complexity compared to RoF (that needs a linearly modulated laser), or even to CPRI (that needs a fast digitalization and IQ packet aggregation). Latency of analog-relaying is negligible and all latency of RoC is just due to propagation; this is the only viable approach that could enable the 1 ms end-to-end latency forecasted in 5G systems. In addition, RUs in RoC can be remotely powered from BBU hotel by using the copper with the same paradigm of power over Ethernet (PoE) standardized by IEEE 802.3 since 2003 with different power levels.
TABLE 1CPRI over fiberRoFRoCBandwidth×18-30×1×1LatencyMedium (several s)Very-lowVery-lowComplexity ofHigh-speed electronicsLow-speedLow-speedcomponents& opticselectronics & optics electronicsLinearity ofRecommendedMandatoryMandatorycomponents(optical(electroniccomponents)components)SynchronizationPacket-levelCarrier frequencyCarrier frequencyand symbolsand symbolsPower supply of RUnonoyes
The first example of RoC was developed for femtocell system with the remotization of the home-antenna and the analog-repetition of the analog radio signal over copper lines is employed by the analog-to-analog (A/A) converter that bi-directionally translates the radio-frequency spectrum to comply with twisted-pair telephone lines [6], or in U.S. Pat. No. 9,107,203. An example of RoC is the RadioDot product by Ericsson [7].
According to [6] summarized in FIG. 2, in the RoC transport over twisted-pair cables, each A/A (20) is connected to one antenna (21), and to one twisted-pair cable (22) acting as front-hauling. The scenario claimed in U.S. Pat. No. 9,107,203 and illustrated here for the context of RU-BBU case is when every RU has one antenna connected to one twisted-pair cable, multiple twisted-pairs are connected to the corresponding RU and all are within the same cable binder (23). RF signals transported over the cables interfere one another due to the electrical coupling among twisted-pairs, this interference is the far-end crosstalk (FEXT). Multiple antennas connecting multiple users with their antennas (called also terminals in jargon) is a Radio MIMO, and the FEXT interfered cables is Cable MIMO. Radio MIMO and cable MIMO compound one another in cascade to yield the radio-cable MIMO (RC-MIMO). Namely, the RC-MIMO is characterized by intra-system interference (i.e., interference arising from the terminals in the same cell or the same carrier frequency, and the cables within the same cable binder), but also inter-system interference (i.e., interference arising by other terminals served by other cells reusing the same carrier frequency, and/or cables using xDSL services coexisting in the same cable binder, or cables too closely spaced apart). Major limitation is that RF signals over cables are mutually FEXT interfering. Since radio access protocols accounts for the adoption of MIMO algorithms as part of the routine processing at BBU, the augmentation of the intra-system FEXT seems not critical for the performance of the overall RC-MIMO system. In other words, at BBU the RC-MIMO is just a more complex MIMO.
In detail, the multicell system of FIG. 2 can be interpreted practically and analytically as the compound of linear operator (matrixes) as sketched in FIG. 2, with matrixes Hair accounting for air MIMO (number of users times number of antennas) and Hcable accounting for cable MIMO (square matrix with the number of cables).
The uplink system model (downlink model would be dual in broadcasting-mode) can therefore be expressed by the following formula:y=HcableBHairs+HcableBnair+ncable wherein ‘y’ is the signal received at a BBU, ‘s’ is the uplink signal received by a RU of the C-RAN, nair the number of RU, ncable the number of cables and B the gain matrix of the analog-to-analog (A/A) converters.
It should be noticed that A/A gains are decoupled one another as each A/A device is accessing to one antenna and it is not exchanging direct information with others; for this reason matrix B is a diagonal one having non-null diagonal elements bi,i representing the gain of the i-th A/A converter. Power shaping of A/A has two benefits: pre-compensate (or post-compensate) for the cable attenuation that has a remarkable frequency-dependent attenuation [8].
In order for A/A devices to mitigate the crosstalk interference between cable and xDSL services and to enable high-datarate services, one can use cables that group a set of twisted-pairs with some shielding and possibly without any coexisting other services [7]. Examples are CAT5 or CAT6 cables, conventionally used for Local Area Networks. Although RoC as front-hauling has shown high potentiality and advantages over RoF or CPRI over fiber, there's a need to control the interference in RC-MIMO to enable the transport of wide radio-spectrum of RF signals to provide improved services to the end-users by carrier aggregation and multiple air-protocols (e.g. WiFi, LTE, UMTS, IEEE802.15 for WSN and domotics, 5G, etc. . . . ), and deploy a number of antennas at RU that are higher than the number of twisted-pair copper lines to each RU.